Field of the Invention
The present invention relates to a process for the preparation of stabilised impact-modified polycarbonate compositions using dilute solutions of acidic compounds, and to the compositions so prepared themselves.
Description of Related Art
The compounding process according to the invention permits the preparation of impact-modified polycarbonate compositions having an advantageous combination of hydrolytic stability and high processing stability, measured by streaking, polycarbonate molecular weight degradation, natural colour and gloss level stability and low-temperature notched impact strength, elongation at tear and stress cracking resistance under the influence of media, in each case considered on components that have been manufactured at high processing temperatures.
In the processing of polymers, compounding refers to the preparation of a finished plastics moulding composition, the compound, from optionally a plurality of polymeric raw materials with the optional addition of polymer additives such as, for example, fillers and reinforcing materials, adhesion promoters, lubricants, stabilisers, etc. Compounding takes place predominantly in kneaders or extruders and comprises the process operations of feeding, melting, dispersing, mixing, degassing and pressure build-up. Compounding is generally followed by solidification of the compound, which is effected by cooling, and granulation thereof.
Emulsion polymers, which are used in polycarbonate compositions as impact modifier, are generally worked up in an acidic medium for the purpose of neutralising polymerisation processing aids having a basic action, such as, for example, emulsifiers. This is necessary in order to ensure adequate heat stability of the compositions because, as is known, basic components tend to cause thermal degradation of the polycarbonate under processing temperatures.
It is often additionally necessary to stabilise polycarbonate compositions containing emulsion graft polymers as impact modifier by addition of acidic additives. That is the case in particular when emulsion graft polymers are used that have been worked up in a basic or insufficiently acidic medium.
Although such compositions known from the prior art generally have good processing stability, they exhibit unsatisfactory stability to hydrolytic cleavage of the polycarbonate under application conditions (for example at temperatures <100° C. and high humidity) and a poor (yellow) natural colour.
EP-A 900 827 describes impact-modified polycarbonate compositions which have improved heat stability and contain emulsion polymers which are substantially free of any basic components that degrade the polycarbonate. According to that application, such polycarbonate compositions impact-modified with emulsion polymers that contain basic impurities from their preparation exhibit unsatisfactory processing stability.
EP-A 576 950 A1 and WO-A 2007/065579 describe compositions containing polycarbonate and acrylonitrile-butadiene-styrene (ABS) polymers, which compositions contain basic impurities and are stabilised with multifunctional organic carboxylic acids. Such compositions have good heat stability with regard to the integrity of the molecular weight of the polycarbonate component at high processing temperatures, but surface defects (streaks) tend to form on the mouldings produced therefrom in injection moulding.
US2006/0287422 describes thermoplastic compositions containing polycarbonate, an impact modifier, optionally a vinyl copolymer, a mineral filler and an acid or an acidic salt, having improved mechanical properties and a reduced tendency to thermal degradation. The application discloses as preferred acids also phosphorus-based compounds of the general formula HmPtOn, specifically inter alia also phosphoric acid. The application discloses that the compositions according to the invention can be prepared by processes described in the prior art.
In WO-A 2010/063381 there are described impact-modified polycarbonate compositions having an improved combination of hydrolytic and processing stability, which compositions contain polycarbonate, an emulsion graft polymer containing basic impurities, and an acidic phosphorus compound having at least one P—OH functionality. As acidic phosphorus compounds there are described both specific cyclic organophosphite compounds and inorganic or organic phosphorus compounds such as, for example, phosphoric acid or phosphoric acid esters.
EP 22 57 590 discloses polycarbonate compositions having an improved combination of natural colour, hydrolytic stability and processing stability, containing polycarbonate, rubber-modified graft polymer containing residues of a fatty acid salt emulsifier resulting from its preparation, wherein the graft polymer in aqueous dispersion has a pH value greater than 7, and an acidic additive. In that application, hydroxy-functionalised mono- and poly-carboxylic acids as well as phosphoric acid are disclosed as the acidic additive.
While it is known from the prior art to add, for example, acidic compounds such as citric acid or phosphoric acid, the use of such acids in polycarbonate compositions which have been prepared by processes described in the prior art frequently leads to disadvantages such as streaking at the surface of components produced from such compositions or pronounced molecular weight degradation as well as unsatisfactory mechanical properties.
None of the mentioned passages describes a process for the preparation of stabilised polycarbonate compositions according to the present invention.